Method of forming carbon nanotubes

ABSTRACT

Provided is a method of forming carbon nanotubes. The method includes modifying the surfaces of catalyst nano particles using a surface modifying agent containing silicon such as 3-aminopropyltriethoxysilane, and growing the carbon nanotubes on the modified surfaces of the catalyst nano particles so that the diameter of the carbon nanotubes is controlled by the surface modification. The diameter of the carbon nanotubes is controlled by surface steric stabilization resulting from a silicon oxide deposit deposited on the surfaces of the catalyst nano particles through the decomposition of the surface modifying agent.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2005-0074473, filed on Aug. 12, 2005, and Korean Patent Application No. 10-2006-0074318, filed on Aug. 7, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to carbon nanotubes, and more particularly, to a method of forming carbon nanotubes in which the diameter of carbon nanotubes grown on catalyst nano particles can be controlled by improving the surface of the catalyst nano particles.

2. Description of the Related Art

Carbon nanotubes can be structurally aeolotropic with differences in diameters from about 1 nm to about 5 nm and lengths from a few μm to a few hundred μm. A carbon nanotube can be a conductor or a semiconductor according to the diameter and chirality thereof. Accordingly, much research into the application of carbon nanotubes in various electronic devices has been conducted.

Carbon nanotubes can be formed using, for example, arc discharge, a laser, carbon monoxide at high temperature and high pressure, or a thermal chemical vapor deposition (thermal CVD) synthesizing method.

The diameter of the carbon nanotubes grown using the CVD method depends on the size of a nano particle catalyst. However, the nano particle catalyst have a very high blaine (a ratio of surface area with respect to volume), and have a low melting point and thus tend to agglomerate at a process temperature used in the CVD method due to its very strong surface free energy. Therefore, the agglomeration of the carbon nanotubes readily occurs at the process temperature. Accordingly, the diameter of the carbon nanotubes can be greater than the particle size of the catalyst nano particles used for growing the carbon nanotubes.

To prevent the agglomeration of the nano particle catalyst, neutral nano particles that physically separate the active catalyst nano particles from each other can be used, or the catalyst nano particles can be embedded in a porous supporter. However, the use of these methods limits the application of the carbon nanotubes.

These methods incorporate a secondary element, for example, the neutral nano particles or the porous supporter, in addition to the catalyst nano particles. Therefore, these methods require a process of removing the secondary element from the synthesized carbon nanotubes. Also, the density of the synthesized carbon nanotubes is reduced since the density of the active catalyst nano particles is reduced due to the presence of the secondary element.

The porous supporter is mainly formed of a metal oxide, and the metal oxide is generally an insulator. However, the carbon nanotubes must be grown on a conductive supporter to be used as an electrical connector. Therefore, there is a need to develop a method that can control the diameter of synthesized carbon nanotubes by preventing the agglomeration of the nano particle catalyst and/or the carbon nanotubes when the carbon nanotubes are growing.

SUMMARY

The present invention provides a method of forming carbon nanotubes in which the diameters of the carbon nanotubes can be controlled by preventing the agglomeration of the nano particle catalyst and/or the carbon nanotubes.

According to an embodiment of the present invention, there is provided a method of forming carbon nanotubes including modifying the surfaces of catalyst nano particles using a surface modifying agent containing silicon, and growing the carbon nanotubes on the modified surfaces of the catalyst nano particles so that the diameter of the carbon nanotubes is controlled by the surface modification.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

FIGS. 1 through 3 are schematic diagrams for explaining a method of forming carbon nanotubes according to an embodiment of the present invention;

FIGS. 4 and 5 are SEM images of carbon nanotubes according to an embodiment of the present invention; and

FIG. 6 is a SEM image of conventional carbon nanotubes.

DETAILED DESCRIPTION

The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough and complete and fully conveys the concept of the invention to those skilled in the art.

In an embodiment of the present invention, a wet chemical synthetic method is used to uniformly control the size of catalyst nano particles when the catalyst nano particles are formed. The wet chemical synthetic method is a method for uniformly growing the catalyst particles to a nano size. The catalyst nano particles used in the present embodiment may have a uniform diameter of, for example, about 3 nm to about 4.5 nm.

In the present embodiment, to control the diameter of the carbon nanotubes growing from the catalyst nano particles, a technique of attaching a surface modifying agent to the surface of the catalyst nano particles is used. A surface modifying agent used in the present embodiment forms a SiO₂ based surface modifying layer on the surface of the catalyst nano particles by decomposing and combining with the catalyst nano particles.

The surface modifying agent can be a molecule that includes silicon (Si), for example, 3-aminopropyltriethoxysilane (APS). The surface modifying agent, such as APS, has an amine group as a terminal functional group. The amine group combines the APS to the catalyst nano particles while the adhesion of the silicon oxide to the surfaces of the catalyst nano particles is controlled by the silane group.

A silicon oxide based layer obtained from the surface modifying agent covers part of the surfaces of the catalyst nano particles, which may reduce the effective surface area of the catalyst nano particles. Therefore, the diameters of the carbon nanotubes can be controlled through the relative reduction of the effective surface area of the catalyst nano particles. When the carbon nanotubes are grown on the surfaces of the catalyst nano particles on which the surface modifying agent is attached, the silicon oxide based layer formed on the surfaces of the catalyst nano particles by the decomposition of the APS molecules substantially controls the diameter of the carbon nanotubes. Accordingly, the diameters of the carbon nanotubes can be controlled to be substantially similar to the diameters of the catalyst nano particles.

FIGS. 1 through 3 are schematic diagrams for explaining a method of forming carbon nanotubes according to an embodiment of the present invention.

Referring to FIG. 1, catalyst nano particles 100 are prepared with a uniformly controlled size of, for example, about 3 nm to about 4.5 nm using a wet chemical synthetic method. The wet chemical synthetic method is a method of forming catalyst nano particles having a uniform size, shape, and composition. The size of the carbon nanotubes depends on the size of the catalyst nano particles 100. Therefore, the uniformity in size of the catalyst nano particles 100 is very important. The particle size distribution of the catalyst nano particles 100 may be controlled to have a standard deviation of less than about 15%.

The catalyst nano particles 100 can be transition metal nano particles. The transition metal nano particles act as a catalyst when carbon atoms are synthesized into carbon nanotubes. An example of the transition metal nano particles is Fe nano particles.

The surfaces of the catalyst nano particles 100 are treated with a surface modifying agent 200. The surface modifying agent 200 can be a molecule containing a silicon (Si) atom, for example, APS.

The molecule can further have a functional group containing a silicon atom, such as a silane group, a siloxane group, or a silsesquioxane group, and may, in addition, have a functional group, such as —NH₂, —COOH, —CONH₂, or —SH. The number of functional groups containing silicon is not limited, and the surface modifying agent 200 can be any molecule that contains at least one functional group containing a silicon atom.

The functional groups such as —NH₂, —COOH, —CONH₂, and —SH allow the surface modifying agent 200 to combine with the catalyst nano particles 100. For example, in the case of APS, while the silane group controls the adhesion of the silicon oxide to the surfaces of the catalyst nano particles, the amine group attaches the APS 200 to the surfaces of the catalyst nano particles 100, for example, through chemical bonding, covalent bonding, or bonding through a weak force such as a van der Waals interaction.

The surface modifying agent 200 can be attached to the surfaces of the catalyst nano particles 100 by directly adding the surface modifying agent 200 in a substantially pure form to a nano particle dispersion solution that is obtained by dissolving the catalyst nano particles 100 in a solvent, or by adding a solution obtained by dissolving the surface modifying agent 200 in an appropriate solvent to a nano particle dispersion solution. After the surface modifying agent 200 is added to the nano particle dispersion solution, a stirring process can be performed so that the surface modifying agent 200 can combine with the catalyst nano particles 100.

Since the surface modifying agent 200 is attached to the surfaces of the catalyst nano particles 100, there are no substantial limitations to the amount of the surface modifying agent 200 added to the nano particle dispersion solution. Accordingly, a sufficient amount of the surface modifying agent 200 can be added to the nano particle dispersion solution.

Referring to FIG. 2, the catalyst nano particles 100 having modified surfaces by being treated with the surface modifying agent 200 are deposited on a substrate 300. At this time, the catalyst nano particles 100 can be deposited on a conductive layer 400 on the substrate 300. This is because the conductive layer 400 is more suitable for using the carbon nanotubes as electrical connectors.

The conductive layer 400 can be a conductive material layer formed of Au, Ag, ITO, TiN, or TaN. In addition, the catalyst nano particles 100 can be directly deposited on the substrate 300, for example, a silicon substrate or a glass substrate, without the conductive layer 400.

After preparing a depositing solution that includes the catalyst nano particles 100 treated with the surface modifying agent 200, the deposition can be performed. The depositing solution can be the nano particle dispersion. The depositing solution can further include a solvent, such as n-hexane, chloroform, toluene, water, and ethanol that disperses the catalyst nano particles 100.

The catalyst nano particles 100 can be deposited on the conductive layer 400 on the substrate 300 by coating the depositing solution (or the nano particle dispersion solution) on the substrate 300 using a solution based coating technique. The coating technique may be spin coating, dip coating, puddle coating, or spray coating.

Since the surface modifying agent 200 is attached to the surfaces of the catalyst nano particles 100, the molecule that constitutes the surface modifying agent 200 can further include a terminal functional group that can combine with the substrate 300 or the conductive layer 400 to facilitate the adhesion of the catalyst nano particles 100 to the conductive layer 400 or the substrate 300. For example, the surface modifying agent 200 can further include a hydrophilic functional group. Since APS is a hydrophilic substance, it can combine with the substrate 300.

Referring to FIG. 3, after the catalyst nano particles 100 having surfaces treated or modified by the surface modifying agent 200 are deposited on the conductive layer 400, carbon nanotubes 500 are grown from the deposited catalyst nano particles 100. The growth of the carbon nanotubes 500 can be performed using a CVD synthetic method, for example, a thermal CVD method or a plasma enhanced CVD method.

The carbon nanotubes 500 are grown on the surfaces of the catalyst nano particles 100 treated with the surface modifying agent 200 by supplying a carbon source gas, for example, acetylene (C₂H₂), at a reaction temperature greater than a decomposing temperature of the carbon source gas, for example, 650° C. When plasma is incorporated, the reaction temperature can be lowered since the plasma promotes the decomposition of the carbon source gas.

The diameter of the growing carbon nanotubes 500 depends on the size of the catalyst nano particles 100. The surface modifying agent 200 combined with the catalyst nano particles 100 can control the diameter of the growing carbon nanotubes 500. The surface modifying agent 200, for example, APS, decomposes to form a deposit 201 of silicon oxide (hereinafter, a silicon oxide deposit 201) on the surfaces of the catalyst nano particles 100.

The amine group of the APS helps the APS attach to the surfaces of the catalyst nano particles 100 while the silicon oxide deposit 201 adheres to the surfaces of the catalyst nano particles 100 via the silane group. As the APS decomposes, silicon of the silane group adheres to the surfaces of the catalyst nano particles 100 to form the silicon oxide deposit 201. The carbon nanotube synthetic reaction cannot progress in the regions where the silicon oxide deposits 201 are formed. In detail, the migration of carbon radical or carbon atoms may be prevented at the surface of the silicon oxide deposit 201, and accordingly, the growth of the carbon nanotubes 500 can be controlled.

Since the effective surface area of the catalyst nano particles 100 is substantially reduced by the adherence of the silicon oxide deposits 201, the aggloeration of the catalyst nano particles and/or the agglomeration of the carbon nanotubes 500 due to an excessive carbon reaction can be prevented. That is, the surface steric stabilization results from the spatial arrangement of atoms or molecules on the surfaces of the catalyst nano particles 100. The presence of the surface modifying agent 200 can prevent the agglomeration of the carbon nanotubes 500 and/or the agglomeration of the catalyst nano particles, and accordingly, the diameter of the carbon nanotubes 500 can be controlled.

Since the agglomeration of the catalyst nano particles 100 is prevented, the carbon nanotubes 500 can be grown with a diameter substantially determined by the size of the catalyst nano particles 100. Accordingly, the carbon nanotubes 500 can have diameters substantially equal to the diameters of the catalyst nano particles 100. Since the catalyst nano particles 100 are formed to a very uniform size using the wet chemical synthetic method, the carbon nanotubes 500 grown on the catalyst nano particles 100 can also have a substantially uniform size.

FIGS. 4 and 5 are SEM images of carbon nanotubes according to an embodiment of the present invention. In detail, FIGS. 4 and 5 are SEM images of carbon nanotubes synthesized using iron catalyst nano particles, the surfaces of which are treated with APS by a thermal CVD method while supplying C₂H₂ at a temperature of approximately 650° C. FIG. 6 is a SEM image of carbon nanotubes synthesized without using APS.

Comparing the SEM images of FIGS. 4 and 5 and the SEM image of FIG. 6, the carbon nanotubes shown in FIGS. 4 and 5, that is, when the catalyst nano particles treated with APS are used, the agglomeration of the nanotubes does not occur as in the conventional nanotubes illustrated in FIG. 6. In other words, the carbon nanotubes illustrated in FIGS. 4 and 5 are grown with more uniform diameters and at a higher density than the carbon nanotubes shown in FIG. 6.

According to an embodiment of the present invention, the surfaces of catalyst nano particles can be modified through a surface treatment with a surface modifying agent containing silicon atoms, for example, APS, and surface steric stabilization is realized on the modified surfaces of the catalyst nano particles when the carbon nanotubes are synthesized. Accordingly, the diameters of the carbon nanotubes are controlled. Furthermore, the carbon nanotubes having a uniform diameter can be grown on a conductive film by preventing the agglomeration of the carbon nanotubes.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. 

1. A method of forming carbon nanotubes, the method comprising: modifying the surfaces of catalyst nano particles using a surface modifying agent containing silicon; and growing the carbon nanotubes on the modified surfaces of the catalyst nano particles so that the diameter of the carbon nanotubes is controlled by the surface modification.
 2. The method of claim 1, wherein the catalyst nano particles include transition metal nano particles having a particle size distribution with a standard deviation of approximately 15% using a wet chemical synthetic method.
 3. The method of claim 1, wherein the surface modifying agent comprises a functional group chosen from a silane group, a siloxane group, and a silsesquioxane group.
 4. The method of claim 1, wherein the surface modifying agent comprises a functional group chosen from —NH₂, —COOH, —CONH₂, and —SH.
 5. The method of claim 1, wherein the surface modifying agent comprises 3-aminopropyltriethoxysilane.
 6. The method of claim 1, wherein the modifying of the surfaces of the catalyst nano particles with the surface modifying agent comprises: preparing a nano particle dispersion solution in which the catalyst nano particles are dispersed; and adding the surface modifying agent to the nano particle dispersion solution directly or after being dissolved in a solution.
 7. The method of claim 1, wherein the growing of the carbon nanotubes comprises performing chemical vapor deposition by supplying a carbon source gas over the catalyst nano particles and decomposing the carbon source gas to synthesize the carbon nanotubes.
 8. The method of claim 7, wherein the chemical vapor deposition is thermal chemical vapor deposition in which the carbon source gas is thermally decomposed or plasma enhanced chemical vapor deposition, in which the carbon source gas is decomposed by plasma.
 9. The method of claim 1, wherein the diameters of the carbon nanotubes is controlled by surface steric stabilization resulting from a silicon oxide deposit deposited on the surfaces of the catalyst nano particles through the decomposition of the surface modifying agent.
 10. A method of forming carbon nanotubes, the method comprising: dispersing catalyst nano particles in a solvent to obtain nano particle dispersion; modifying the surfaces of the catalyst nano particles in the nano particle dispersion by adding a surface modifying agent containing silicon to the nano particle dispersion; coating the nano particle dispersion on a substrate; and growing the carbon nanotubes on the catalyst nano particles coated on the substrate so that the diameter of the carbon nanotubes is controlled by the surface modification.
 11. The method of claim 10, wherein the solvent includes at least one of n-hexane, chloroform, toluene, water, and ethanol.
 12. The method of claim 10, wherein the surface modifying agent comprises a functional group chosen from a silane group, a siloxane group, and a silsesquioxane group.
 13. The method of claim 10, wherein the surface modifying agent comprises a functional group chosen from —NH₂, —COOH, —CONH₂, and —SH.
 14. The method of claim 10, wherein the coating of the nano particle dispersion on the substrate is performed using a coating method selected from at least one of a spin coating, a dip coating, a puddle coating, and a spray coating.
 15. The method of claim 10, wherein the substrate is a glass substrate, a silicon substrate, or a substrate having a layer that comprises a conductive material chosen from Au, Ag, ITO, TiN, and TaN.
 16. The method of claim 10, wherein the diameter of the growing carbon nanotubes is controlled by surface steric stabilization resulting from a silicon oxide deposit deposited on the surfaces of the catalyst nano particles through the decomposition of the surface modifying agent.
 17. A method of forming carbon nanotubes, the method comprising: dispersing catalyst nano particles in a solvent to obtain nano particle dispersion; modifying the surfaces of the catalyst nano particles in the nano particle dispersion by adding a surface modifying agent containing silicon and an amine functional group to the nano particle dispersion; coating the nano particle dispersion on a substrate; and growing the carbon nanotubes on the catalyst nano particles coated on the substrate so that the diameter of the carbon nanotubes is controlled by the surface modification.
 18. The method of claim 17, wherein the surface modifying agent comprises 3-aminopropyltriethoxysilane.
 19. The method of claim 17, wherein the surface modifying agent comprises a functional group chosen from a silane group, a siloxane group, and a silsesquioxane group.
 20. The method of claim 17, wherein the diameter of the growing carbon nanotubes is controlled by surface steric stabilization resulting from a silicon oxide deposit deposited on the surfaces of the catalyst nano particles through the decomposition of the surface modifying agent.
 21. A method of forming carbon nanotubes, the method comprising: providing catalyst nano particles; treating surfaces of catalyst nano particles with a surface modifying agent containing silicon; and growing the carbon nanotubes on the surfaces of the catalyst nano particles treated with the surface modifying agent containing silicon.
 22. The method of claim 21, wherein a diameter of the carbon nanotubes is controlled by treating the surfaces of catalyst nano particles.
 23. The method of claim 21, wherein the catalyst nano particles comprise Fe nano particles. 